Investigating Alkoxysilane Coverage and Dynamics on the (104) and (110) Surfaces of MgCl 2 ‑Supported Ziegler-Natta Catalysts Raffaele Credendino, † Jochem T. M. Pater, ‡ Dario Liguori, ‡ Giampiero Morini, ‡ and Luigi Cavallo* ,†,§ † Chemical and Life Sciences and Engineering, Kaust Catalysis Center, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia ‡ LyondellBasell Polyolefins, G. Natta Research Center, P. le G. Donegani 12, 44100 Ferrara, Italy § Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX Eindhoven, The Netherlands * S Supporting Information ABSTRACT: In this work, we present a systematic DFT analysis of the effect of surface coverage on the coordination properties of alkoxysilanes to the (104) and (110) surfaces of MgCl 2 . Furthermore, we investigated several possible migration pathways for alkoxysilane migration on the same surfaces. Our study clearly shows that complete coverage of the Mg vacancies on the surface by coordinating alkoxysilanes is hampered by steric repulsion between vicinally coordinated donor molecules. Our study clearly indicates that alkoxysilane migration between different MgCl 2 monolayers on the (104) and (110) surfaces requires donor dissociation. The same holds for alkoxysilane migration on a single (110) MgCl 2 monolayer. However, in the case of the (104) surface we found a very low energy pathway for alkoxysilane migration along the same monolayer. ■ INTRODUCTION Heterogeneous Ziegler-Natta (ZN) catalysts are the most important catalysts in the industrial production of isotactic polypropylene. The typical catalysts used are MgCl 2 /TiCl 4 / donor systems where the donor is a Lewis base (LB) that can be added during catalyst preparation (the so-called internal donor, ID) or during activation (the so-called external donor). 1 Alkoxysilanes, 1,3-diethers, aromatic esters (benzoates and phthalates in particular), and recently aliphatic esters (succinates in particular) were shown to be particularly effective donors. 1 The resulting active system possesses extreme chemical complexity, and the polypropylene that is obtained presents very different properties depending on the specific components and recipe used in the preparation. Focusing on the role of the LB is fundamental in the overall catalyst performance because it can significantly impact (i) the microstructure of the obtained polypropylene; (ii) the molecular mass distribution; and (iii) the response to molecular hydrogen, and it can also have an impact on the morphology of the catalyst because they can stabilize small primary crystallites of MgCl 2 and/or influence the amount and distribution of TiCl 4 in the final catalyst. 2-16 The characterization of heterogeneous Ziegler-Natta catalysts has been the subject of several studies, which underlines the difficulties inherent in the detailed understanding of these catalysts. 13,15-36 Nevertheless, these studies allowed us to clarify several points that are now well accepted. For example, it is clearly accepted that the primary particles of activated MgCl 2 are composed of a few irregularly stacked Cl-Mg-Cl sandwichlike monolayers. 37 These MgCl 2 layers should be terminated by the (104) and (110) lateral cuts 24,38 that contain coordinatively unsaturated Mg 2+ ions with coordination numbers of 4 and 5 on the (110) and (104) cuts, respectively, as shown in Figure 1. 38,39 The problems start with the quantification of the relative numbers of (104) and (110) lateral cuts, which of course also depends on the recipe used for catalyst preparation. MgCl 2 monolayers forming the (104) lateral cut were suggested to be more stable than the (110) lateral cut because of the lower unsaturation of the surface Mg atoms on the (104) monolayer. A clear quantification of this old concept was provided by Busico and co-workers, who used periodic DFT calculations to Received: August 31, 2012 Revised: October 3, 2012 Published: October 3, 2012 Figure 1. Schematic representation of a MgCl 2 crystallite presenting (104) and (110) lateral cuts. Article pubs.acs.org/JPCC © 2012 American Chemical Society 22980 dx.doi.org/10.1021/jp308658c | J. Phys. Chem. C 2012, 116, 22980-22986